Turbine engines are used as the primary power source for aircraft, in the forms of jet engines and turboprop engines, as auxiliary power sources for driving air compressors, hydraulic pumps, etc, on aircraft, industrial gas turbine (IGT) power generation and as stationary power supplies such as backup electrical generators for hospitals and the like. The same basic power generation principles apply for all these types of turbine engines. Compressed air is mixed with fuel and burned, and the expanding hot combustion gases are directed against stationary turbine vanes in the engine. The vanes turn the high velocity gas flow partially sideways to impinge on the turbine blades mounted on a turbine disk or wheel that is free to rotate.
The force of the impinging gas causes the turbine disk to spin at high speed. Jet propulsion engines use this power to draw more air into the engine and the high velocity combustion gas is passed out of the aft end of the gas turbine, creating forward thrust. Other engines use this power to turn a propeller, electrical generator, or other devices.
High pressure turbine (HPT) components, including turbine blades are critical components in any turbine engine. During operation of the turbine engine, the HPT components are subjected to high heat (often in excess of 2000 degrees F.) and stress loadings as they experience operational conditions and are impacted by the hot gas. This high heat and stress can result in unacceptably high rates of degradation on the turbine components due to erosion, oxidation, corrosion, thermal fatigue cracks and foreign object damage. Such conditions result in many cases in the need for repair and/or replacement, something that can result in significant operating expense and time out of service.
Traditional methods of repair have had limited success. One primary reason for the lack of success is that the materials used to make HPT components do not lend themselves to conventional repair techniques. For example, many materials currently used in turbine blades and vanes suffer from poor weldability. Repairing the turbine blade with conventional welding techniques subjects the turbine blade to high temperatures. However, at such high temperatures the welding areas are likely to suffer severe oxidation. Also, repairing the HPT components with conventional welding techniques at room temperature is prone to form hot cracking in the welding area. This can require extensive reworking thus adding significantly to the cost of the repair. Furthermore, in some cases the cracking can cause the repair to be ineffective or otherwise render the turbine blade unusable for further engine service.
As one specific example, in the repair of turbine blade tips current extension repair techniques have included the application of intermediate cladding of materials which exhibit good weldability to the turbine blade prior to welding the final tip extension. Unfortunately, this method also has limitations in that adding the cladding material can itself create stress and defects in the turbine blades. Furthermore this method can create performance mismatches between those turbine blades that have been repaired and those that have not been repaired.
Thus, there is a need for the development of new repair methods that improve the reliability and performance of the repair at cheaper costs in the refurbishment process.